Various embodiments described herein relate generally to the field of heating, ventilation, and air conditioning (HVAC), and more particularly to HVAC systems having distributed zone control units that locally re-circulate air within zones serviced by the zone control units. Such an HVAC system may be particularly effective for use in office building, hospitals, hotels, schools, apartments, research labs, multi-family residences, and single-family residences.
A range of approaches are used in existing HVAC systems. Existing HVAC systems include, for example, conventional forced air variable volume systems and systems employing chilled beams.
Conventional Forced Air Variable Air Volume Systems
A conventional forced air variable air volume (VAV) system distributes air and water to terminal units installed in habitable spaces throughout a building. The air and water are cooled or heated in central equipment rooms. The air supplied is called primary or ventilation air. The water supplied is called primary or secondary water. Steam may also be used. Some terminal units employ a separate electric heating coil in lieu of a hot water coil. The primary air is first tempered through a large air handling unit and then distributed to the rest of the building through conventional air duct work. The large air handling unit may consist of a supply fan, return fan, exhaust fan, cooling coil, heating coil, filters, condensate drain pans, outside air dampers, return dampers, exhaust dampers, sensors, controls, etc. Once the primary air leaves the air handling unit the primary air is distributed through out the building through air duct work and then to in-room terminal units such as air distribution units and terminal units. A single in-room terminal unit usually conditions a single space, but some (e.g., a large fan-coil unit) may serve several spaces. Air distribution units and terminal units are typically used primarily in perimeter spaces of buildings with high sensible loads and where close control of humidity is not desired; they are also sometimes used in interior zones. Conventional forced air variable air volume systems work well in office buildings, hospitals, hotels, schools, apartments, and research labs. In most climates, these VAV systems are typically installed to condition perimeter building spaces and are designed to provide all desired space heating and cooling, outside air ventilation, and simultaneous heating and cooling in different parts of the building during intermediate seasons.
A conventional forced air variable air volume system has several disadvantages. For example, because large volumes of air circulated around a building, fan energy consumption and temperature losses may be significant. To minimize energy consumption, the large air handling unit may recycle the circulated air and only add a small portion of fresh air. Such recycling, however, may result in air borne contaminants and bacteria being spread throughout the building resulting in “sick building syndrome.” Other disadvantages may include draughts, lack of individual control, increased building height required to accommodate ducting, and noise associated with air velocity. Additionally, for many buildings, the use of in-room terminal units may be limited to perimeter spaces, with separate systems required for other areas. More controls may be needed as compared to other systems. In many systems, the primary air is supplied at a constant rate with no provision for shut off, which may be a disadvantage as tenants may prefer to shut off their heating or air conditioning or management may desire to do so to reduce energy consumption. In many systems, low primary chilled water temperature and or deep chilled water coils are required to control space humidity accurately, which may result in more energy consumption from a chiller, cooling tower, and/or pumps. A conventional forced air variable air volume system may not be appropriate for spaces with large exhaust requirements such as labs unless supplementary ventilation is provided. In many systems, low primary air temperatures require heavily insulated ducts. In many systems, the energy consumption is high because of the power needed to deliver primary air against the pressure drop of the terminal units. The initial cost for a VAV system may be high. In many systems, the primary air is cooled, distributed, and may be subsequently re-heated after delivery to a local zone, thus wasting energy. In many systems, individual room control is expensive as an individual terminal unit or fan coil unit is required for each zone, which may be costly to install and maintain, including for ancillary components such as controls. Moving large flow rates of air thru duct work is inefficient and wastes energy. Mold and biocides may form in the duct work and then be blown into the ambient/occupied space.
Chilled-Beam Systems
A chilled beam uses water, not air, to remove heat from a room. Chilled beams are a relatively recent innovation. Chilled beams work by pumping chilled water through radiator like elements mounted on the ceiling. As with typical air ventilation systems, chilled beams typically use water heated or cooled by a separate system outside of the space. The building's occupants and equipment (e.g., computers) heat the air, which rises and is cooled by the chilled beam creating convection currents. Radiant cooling of interior elements and exposed slab soffit enhances this convective flow. Room occupants are also cooled (or warmed) by radiant heat transfer to or from the chilled beam.
Chilled beams, however, have some disadvantages. For example, they are relatively expensive due to the use of copper coils. A chilled beam is not easy to relocate, which may require major renovation for some office space reconfigurations. They can also be expensive to install for a variety of reasons, for example, their weight may be an issue with regard to seismic codes; they may take several tradesmen to install; they may require increased piping, valves, and controls compared to other systems; and three to four chilled beams may be required for every VAV air distribution unit or fan coil unit. Air still needs to be tempered to prevent condensation from forming on the chilled beam. They may be unable to provide the indoor comfort required in large spaces. They are exposed directly to the ambient space, which may result in condensate forming on the chilled beam and dripping on to products and equipment below. Substantially unrestricted airflow to the beam is typically required. A chilled beam requires more ceiling area than diffusers of a conventional system, thus leaving less room for sprinklers and lights. This can impact the aesthetics of the interior spaces and require a higher level of coordination for other systems such as lighting, ceiling grid, and fire protection. Mechanical contractors may not be familiar with chilled beams and may charge more. Re-circulated air passing through the chilled beam is not filtered as it would be in a VAV system. A chilled beam may not be suitable for use in an area with a high latent load. Areas such as conference rooms, meeting rooms, class rooms, restaurants, or theaters with dense population may be difficult to condition with chilled beams. Portions of a building that are open to the outside air typically cannot be conditioned with chilled beams. Noise may be an issue with chilled beams due to the use of pressure nozzles, which are factory set for a certain performance, derivation from which causes noise thereby limiting the options of the building occupants. The building should have a very tight construction for humid climates. Naturally ventilated buildings may need to include a sensor to measure dew point in the space and/or window position switches that automatically raise the cooling water temperature or shut down flows to the chilled beam when high dew points are reached. Chilled beams may need to be vacuumed every year. More control valves, strainers, etc. may be desired. Typical room design temperature for chilled beams is 75 to 78 degrees F., which may be too high for healthcare and pharmaceutical applications. A chilled beam typically does not provide a radial-symmetric airflow pattern like most hospital/lab air diffusers; instead, they drive the air laterally across the top of the room, which can disrupt hood airflow patterns.
In light of the above, it would be desirable to have improved HVAC systems and components with increased advantages and/or decreased disadvantages compared to existing HVAC systems and components. In particular, improved HVAC systems and components having reduced installed cost, improved controllability, decreased energy usage, increased recyclability, increased quality, increased maintainability, decreased maintenance costs, and decreased sound would be beneficial.